Conventionally, there is available a trolley system including a vehicle, such as a travelling hoist or a transfer mover, and a power supply device for supplying electric power to the vehicle. In the power supply device, electric power is exchanged between a power supply line arranged along a vehicle-travelling rail and a power receiver provided in the vehicle. The electric power received by the power receiver is supplied to the vehicle. One example of the power supply line is disclosed in Patent Document 1.
FIG. 12 is a perspective view showing the outward appearance of a power supply line for high-frequency current disclosed in Patent Document 1. FIG. 13 is a vertical section view showing a modified example of the power supply line shown in FIG. 12, which employs another conductor formed by extruding copper. As shown in FIGS. 12 and 13, the power supply line for high-frequency current 100 includes a two-layered tubular conductor 200 embedded in an insulating body 300. The conductor 200 includes an inner tube portion 200a and a concentric outer tube portion 200b one-piece connected to the inner tube portion 200a by a connecting portion 200c over the longitudinal full length of the conductor 200. The insulating body 300 is not arranged in the spatial portions 400a and 400b of the respective tube portions 200a and 200b. 
In the example shown in FIG. 12, a conductor 200 is formed by, e.g., bending a single copper plate. More specifically, an inner tube portion 200a is formed by bending the central portion of a plate into an annular cross-sectional shape. Two planar piece portions extending downward in FIG. 12 from the opposite ends of the annular portion forming the inner tube portion 200a are formed in a parallel-extending contact relationship with each other. An outer tube portion 200b of annular cross-sectional shape concentric with the inner tube portion 200a is formed by bending the planar piece portions into an arc shape to surround the inner tube portion 200a, bringing the ends of the arc-shaped bent portions into contact with each other and welding the ends of the arc-shaped bent portions together. The two planar piece portions formed in a parallel-extending contact relationship make up a connecting portion 200c for interconnecting the inner and outer tube portions 200a and 200b. 
In a trolley system, power supply lines are fixed in place by a line hanger 500 as shown in FIGS. 14 and 15. FIG. is a perspective view showing a state that two power supply lines 101 are fixed to a conventional line hanger 500. FIG. 15 is a front view of the line hanger 500 shown in FIG. 14. As shown in FIGS. 14 and 15, the line hanger 500 is used to fix the power supply lines 101 having a circular cross-sectional shape. The line hanger 500 is formed into a substantially U-like shape and includes a pair of holding members 501 and 502 for holding a pair of power supply lines 101 arranged in parallel and a connecting portion 503 for interconnecting the base end portions of the holding members 501 and 502. In the tip end portions of the holding members 501 and 502, there are formed recess portions 501H and 502H for holding the power supply lines 101. The recess portions 501H and 502H are formed into a shape conforming to the outward shape of the power supply lines 101, i.e., the cross-sectional shape of sheaths 301 of the power supply lines 101. Thus, the recess portions 501H and 502H can hold the power supply lines 101 in a closely contacted state with no looseness.
FIG. 16 shows the recess portion 501H (or 502H) of the holding member 501 (or 502) shown in FIG. 14 and the power supply line 101 held in the recess portion 501H (or 502H). As shown in FIG. 16, step-like stoppers 501Ha (or 502Ha) are formed inside the recess portion 501H (or 502H) of the holding member 501 (or 502). The power supply line 101 is locked by the stoppers 501Ha (or 502Ha) and is prevented from being removed with ease.
[Patent Document 1]
Japanese Patent Application Publication No. 2008-117746
However, the power supply line for high-frequency current disclosed in Patent Document 1 suffers from the following problems.
(1) Since the inner tube portion and the outer tube portion are connected by the single connecting portion, the positioning of the inner tube portion becomes unstable and the alternating current resistance tends to increase. In this regard, the high-frequency resistance becomes smallest when the inner and outer tube portions are concentric with each other.
(2) A higher level of technique and an increased cost are required to form the inner tube portion, the outer tube portion and the connecting portion using a single copper plate.
(3) Copper is harder than aluminum, poor in extrusion formability (namely, throughput) and expensive.
The line hanger set forth above suffers from the following problem. Despite the fact that the step-like stoppers are formed in the recess portion of the holding member of the line hanger, the power supply line having a sheath of circular cross-sectional shape is easily removed upward from the holding member.
Since the holding member of the line hanger is not provided with a structure for restraining the power supply line from rotating in the circumferential direction, a problem is posed in that the power supply line is rotated when installed or repaired, which makes it difficult to keep the power supply line in position.